Spatial light modulator devices (SLMs) are used in several areas where the control of light on a pixel-by-pixel basis is needed. For example, SLMs have been used in optical data processing, adaptive optics, optical correlation, machine vision, image processing, image analysis, beam steering, displays, and holographic data storage systems. Many different types of SLMs exist for these and other applications.
In general, an SLM includes a set of controllable optical elements that define a pixel array. The SLM receives input light, and each of the optical elements manipulate a portion of the input light to define the state of a given pixel. For example, each of the optical elements of conventional SLMs can be controlled to be in an “on” state or an “off” state. By controlling which optical elements are “on” and which are “off,” the output of the SLM can be encoded on a pixel-by-pixel basis.
Conventional SLMs are typically either reflective-mode SLMs or transmissive-mode SLMs. In conventional reflective-mode SLMs, the controllable optical elements comprise specular mirrors. In that case, the orientation of the mirrors can be controlled to define the on/off state of pixels, based on the direction that output light is directed from the different mirrors. In conventional transmissive-mode SLMs, the set of elements that can be made transmissive or opaque in order to either pass or block light and thereby define the on/off state of the respective pixels. Machine words may be defined for certain pixel arrays or subsets of the pixel arrays.
For holographic data storage, SLMs are used to encode pixel arrays that comprise bit maps into an object beam of the system. In particular, when input light for the object beam illuminates the SLM, the SLM can encode information into the object beam by controlling which pixels are “on” and which pixels are “off.” The encoded object beam is then made to interfere with a reference beam to record a hologram in the medium. When a stored hologram is later illuminated with only the reference beam, some of the reference beam light is diffracted by the hologram interference pattern. Moreover, the diffracted light can be directed to reconstruct the original encoded object beam. Thus, by illuminating a recorded hologram with the reference beam only, the data encoded in the object beam can be reconstructed and detected by a data detector such as a camera or other image capture device. In this manner, information stored in a recorded hologram can be read from a holographic medium.